1. Field of the Invention
The invention relates generally to telephone central equipment and more particularly to a line interface circuit between telephonic signal originating lines and output tip and ring lines.
2. Description of the Prior Art
The telephone system art has advanced steadily from the earliest days of local multi-party arrangements employing local "talking battery" and ringing magneto. The advent of early manual central switchboards in which subscriber lines and trunk circuits were connected to jacks on a panel receiving plug and cable connections brought about the terms, tip, ring and sleeve. The tip and ring corresponded to the subscriber transmission leads or conductors, i.e. the two wire telephone line. Although the terminology is archaic in terms of present day solid state automatic telephone systems, the terms tip and ring are still used to identify the two conductors of a two wire telephone line.
In relatively modern prior art telephone central equipment, the ringing function is usually implemented by switching the two subscriber lines to a common ringing signal source which is a high voltage (high at least in telephone system terms, i.e. up to 120 volts RMS). The switching of this signal onto the tip and ring lines is conventionally accomplished by means of relays. Those relays essentially isolate the speech and other signals from the line while the ringing voltage is connected and vice versa. The advent of solid state telephone system switching has made possible the compact, reliable and relatively inexpensive implementation of telephone central main frame switching without the great banks of stepping relays and other electromechanical devices. More gradually, improvements are being made in subscriber line/trunk interface circuitry. U.S. Pat. No. 4,161,633 entitled "Subscriber Line/Trunk Circuit" describes certain of these improvements. In particular, a microprocessor controlled interface circuit placed between analog telephone lines and trunks and a digital switching system is disclosed. All processing of incoming analog systems is accomplished by a line circuit including measuring, monitoring, testing functions and the speech path. That reference further shows the generation of a d.c. signal powering the handset microphone as well as ringing and other tones. Certain subsystems in an overall system according to the invention are also shown in U.S. Pat. No. 4,161,633, a form of the so-called programmable signal generator being one of these.
The manner in which the present invention advances the state of this art will be understood as this description proceeds.